This invention relates to a foldable boom, truss, or longeron member, collapsible trusses and other similar structures made of such members.
Key optical components of large aperture, space based optical instruments may be deployed on orbit to provide an aperture large enough to increase the resolution and optical performance by several orders of magnitude. The performance of such instruments depends on maintaining the precision and stability of the deployed structural geometry to within nanometers of an ideal shape. Nonlinear contact mechanics and freedom in the components of deployed structures mean that deployed instruments will have the capacity to change shape at the micron and nanometer level of resolution. Eliminating such nonlinearities as load path friction and freeplay would enable a deployed structure to be as linear and precise as a monolithic block of material.
In most mechanically deployed structures, components are moved from their stored positions into their final operational positions by some type of actuator and then locked into place with a deployment latch. For high precision structures, it is critical that the load paths and load predictable for the reliable operation of the instrument.
Existing deployable structure joints have several limitations that either completely prevent them from being used in high precision deployable instruments or require complex analysis and additional launch mass to provide deployment actuation and post deployment locking. Hinge joints previously used in moderate precision structures have relied on high levels of preload and friction to eliminate freeplay and geometric ambiguity. These joints have been shown to be unstable at the micron level, causing the structure to xe2x80x9cmicro-lurchxe2x80x9d or change shape and thus move the instrument""s optics far out of alignment.
Existing joints for precision space structures relied on high levels of preload between the many components to eliminate gaps and free play that cause inaccuracies in the structure. Unfortunately, these high levels of preload introduce correspondingly high levels of friction both during the deployment and after deployment has been completed. Friction mechanisms are nonlinear and thus are more difficult to control and less predictable.
Other hinge designs such as latch and actuator type systems suffer from the same disadvantages.
Recently, foldable truss members have been developed so that a truss structure can be collapsed and compactly packaged to save space during delivery and then released to expand and return to its original shape in orbit. All of these mechanisms add to the mass, expense and complexity of the structure and to the difficulty and expense of transporting it. These foldable members reduce the mass (and the delivery cost) of the structure by replacing the hinge, latch and actuator mechanisms with one single device. See, e.g., U.S. Pat. No. 4,334,391 incorporated herein by this reference.
Solid rods are joined on their ends forming a truss structure (a square frame for a solar panel array or a superstructure for a communications satellite antenna, for example) and pre-selected rods are cut in sections to form a hinge between the two sections. The rod sections are joined with spring steel elements similar to if not actually lengths of a carpenter""s tape measure.
The rod sections can be folded with respect to each other by imparting a localized buckling force to one of the spring steel elements. Simply letting go of one rod section, returns the two rod sections to an end to end alignment due to the potential energy stored in the biased spring steel hinge elements.
In this way, a truss structure made up of several of these foldable rods can be designed on earth, collapsed for delivery to space, and then released once in position in space where the foldable rods flex back into position forming the truss structure designed and constructed on earth.
In use, this spring steel hinge design suffers from a number of shortcomings.
First, hinges formed of spring steel elements require joining the ends of each spring steel element to a rod section. These joints and the spring steel elements themselves add significantly to the overall weight of the truss structure which is an undesired factor in space launch capability.
The spring steel elements also result in dimensionally unstable truss structures. The dimensional instability is caused by the relative motion of the internal components including the joints between the spring elements and the rod sections and permanent yielding of different areas of the spring elements themselves.
The result is that the shape of the truss structure may change when it is erected in space from the shape of the truss structure before it was collapsed on earth. This can have disastrous effects on instrument performance as even a ten nanometer to ten micrometer displacement can severely affect the performance of primary and secondary optics attached to the truss structure.
It is therefore an object of this invention to provide a foldable member and a collapsible structure made of a number of foldable members that is lighter than prior art foldable members and collapsible structures.
It is a further object of this invention to provide such a member and such a structure which is more dimensionally stable.
It is a further object of this invention to provide such a foldable member which is a single piece design thereby eliminating numerous sources of imprecision.
It is a further object of this invention to provide such a member and such a structure which eliminates the need for deployment actuators and mechanical latches to further reduce system mass.
It is a further object of this invention to provide such a member and such a structure which have tailorable thermal expansion and conductivity properties and which thus can be designed for a multitude of performance requirements.
It is a further object of this invention to provide such a member which can be made of a variety of different types of materials.
It is a further object of this invention to provide such a member which is simple to manufacture and use.
It is a further object of this invention to provide a collapsible tube useful in variety of applications.
This invention results from the realization that a lighter and more dimensionally stable, foldable member can be constructed by cutting or forming longitudinal slots in a tube around the perimeter thereof at a location where the member is designed to bend thereby forming separated, longitudinal strips of tube material at that location which easily buckle allowing the member to fold without adding a separate hinge which would add weight to the member and would also result in dimensional instability.
The foldable member of this invention includes an integral hinge made of the same material that the tube is made of resulting in sharp weight reduction and improved dimensional stability that is especially well suited for space applications.
This invention features a foldable member comprising a tube, at least one predetermined hinge area along the length of the tube, and opposing sets of elongated slots in the tube at the hinge area thereof forming separated longitudinal strips of tube material between the slots which fold when subjected to localized buckling forces.
The opposing sets of elongated slots may be diametrically opposed from each other on the tube and each slot in each set of slots may be diametrically opposed from a slot in the opposing set of slots. A stress relieving member may be attached to each bridge element on the inside of the tube. The tube may be made of a plastic (e.g., polycarbonate) material, a metal material, or made of a composite material such as a triaxial braid of fibers in the shape of a tube embedded in a resin matrix.
Each truss member typically has a plurality of hinge areas spaced from each other along the length of the tube, each hinge area including opposing sets of elongated slots.
This invention also features a collapsible structure comprising a plurality of joined members. A selected number of the members each include a tube, at least one predetermined hinge area along the length of the tube and opposing sets of elongated slots in the tube at the hinge area thereof forming separated longitudinal strips of tube material between the slots which fold when subjected to localized buckling forces, each slot of each set of elongated slots separated longitudinally along the length of the tube from each adjacent slot by a bridge element of tube material.
An electrical conductor may be disposed in the tube to transmit electrical signals about the structure. A transducer device may be located proximate a hinge area for controlling the folding of the longitudinal strips of tube material. Slot reinforcing members may be disposed in the slots. The elongated slots may be triangle or diamond shaped. There may be four slots in each set of slots, each slot of a pair of the four slots opposing another slot. The slots may have a reduced diameter portion.
A second tube may also be disposed inside the first tube. The second tube may, like the first tube, include opposing sets of elongated slots at the hinge area thereof